1. Field of the Invention
The present invention relates to improvements in the performance of implantable defibrillators, implantable cardioverter-defibrillators (ICDs) and other battery powered medical devices designed to provide high energy electrical stimulation of body tissue for therapeutic purposes.
2. Description of Prior Art
High energy battery powered medical devices, such as implantable defibrillators and ICDs, are designed to produce a strong electrical shock to the heart when called upon to correct the onset of tachyarrhythmia. The shock is produced by one or more energy storage capacitors that have been charged by the device's battery power source. This power source is typically a lithium/silver vanadium oxide (Li/SVO) battery or cell of the type disclosed in U.S. Pat. No. 5,458,997 of Crespi, and references cited therein. Crespi notes that the Li/SVO cell chemistry is useful for defibrillation applications because of its ability to produce pulses of energy that can charge the high voltage capacitors within the short time frame required by the device. In particular, the Li/SVO battery is typically called upon to charge the capacitors to deliver within 10 seconds or less a shock of up to 40 Joules. This must be done several times in succession if additional shocks are required. Unfortunately, as noted by Crespi, an Li/SVO cell can experience unpredictable resistance increase upon long-term discharge during service. In particular, Li/SVO cells commonly have a two-stage run down with slightly different voltage plateaus at each stage. It is at the interval between the two plateaus where it is common to see the resistance increase described by Crespi. The problem is further explained in U.S. Pat. No. 6,426,628 of Palm et al. as a being transient phenomenon that occurs following a period of low current draw. When a load is reapplied (e.g., a defibrillation pulse is required), the resistance build-up temporarily prevents the cell from developing its full open circuit voltage potential. This condition, which is referred to as “voltage delay,” continues for a brief period until the resistance diminishes back to some nominal level.
In many cases, the voltage delay experienced by an Li/SVO cell is significant enough to impair the cell's ability to charge the capacitors of a defibrillator or ICD in a timely manner. This may result, prematurely, in a decision being made that the Li/SVO cell has reached end of service (EOS) and needs to be explanted for replacement. In addition to the patient inconvenience and risk entailed by this procedure, a significant portion of the capacity of the Li/SVO cell is needlessly rendered unavailable for long-term use. Even if it is not removed, the cell's operation is unpredictable, thus making any attempt to calculate the EOS point rather complicated.
A need therefore exists for an improvement in defibrillator/ICD battery power systems so as to mitigate the aforementioned characteristics of Li/SVO cells. What is required is an implantable power source with increased stability and whose operation is more predictable as a basis for simple EOS determination.